This invention relates to an improvement in a non-contact radiation thickness gauge which is used to measure the thickness of various materials by detecting the amount or intensity of the radiation transmitted through the material. One such gauge is disclosed in U.S. Pat. No. 3,955,086 to Tsujii et al. and assigned to the present assignee.
Thickness gauges of this type have been used in various applications. Such gauges make it possible to continuously measure the thickness of sheets of various materials without actual contact with the material. The response of radiation gauges, such as x-ray gauges, is fast and can be used for on-line measurement in rolling mill lines of metals where rapid automatic adjustment is necessary to achieve continuous and uniform thickness of the sheets.
The non-contact type thickness gauge found in the prior art normally uses movable standards of predetermined thickness values for calibration of the gauge system. Typically, one or more of the standards of known precise thickness are selected and placed into the beam and an analog meter coupled to the output of the detector is nulled to the particular nominal thickness of the standard. Deviations from the nominal thickness as the material is measured will appear as deviations on the meter. Other prior art systems use complementary standards in which the full measurement range is divided into a plurality of subranges. The calibration for each subrange is normally referenced to a single standard of maximum thickness. This single standard is termed the "base standard." If the gauge is calibrated to null when the base standard is in the path of the beam, it is then possible, with the base standard removed, to measure lesser thicknesses of material which fall within the subrange of thicknesses. Certain ones of the standards which complement the desired thickness of the strip so that the total material thickness is equivalent to that of the base standard are then inserted into the beam. When the thickness is correct, the analog meter will be nulled.
Two point calibration systems have been used. In this type of system, two sets of standards are sequentially inserted into the radiation beam path during the calibration operation. The first of the standards is selected to be the apparent thickness of the material which is calculated from the nominal thickness and alloy compensation coefficient of the material. The apparent thickness of a second standard is selected to be the desired apparent thickness of the material plus some predetermined deviation from the desired thickness. Such a system is calibrated based on the two apparent thickness points and relies on the assumed relation that the output signal of the detector and the thickness of the material to be gauged is linear. Such a system may be considered a linear interpolation system.
In the prior art systems, the calibration is usually done directly for the material to be gauged, i.e., the system is calibrated for apparent thickness. The alloy compensation is accomplished in the calibration process and not during measurement. Such systems also require recalibration each time there is a change of nominal thickness when the thickness of the material to be gauged in the radiation path is changed.
Because such systems usually must combine standards to achieve the apparent thickness of the material to be gauged, a very large number of standard plates must be used. A large number of plates also is required because of the type of linear approximation used in calibrating the system. For instance, in one system a binary coded decimal series of plates starting with a very thin 0.001 mm plate up to a plate of 8 mm thickness is used.
Also in prior art gauging systems, drift in the measurements due to changes in radiation source voltage or sensitivity of the detector must be compensated for. This is usually done by adjusting the gain of a preamplifier through various feedback systems.